The Sentinel rebreather takes a three pronged attack at the carbon dioxide monitoring issue.
First, it has a thermal profile monitor that measures the temperature in various levels of the scrubber bed to show where the effective reaction zone is happening. Secondly, the unit measures the amount of oxygen passing through the solenoid valve and uses that to estimate the amount of carbon dioxide that is generated, essentially giving the diver an idea of how their workload is affecting their scrubber on a given dive. The crown jewel is the gaseous carbon dioxide monitor.
Recently a diver told me he was going to try to take a Sentinel carbon dioxide sensor and fit it into another rebreather. I let him know that the carbon dioxide sensor itself is actually nothing special. It is a rather basic infrared carbon dioxide sensor with some hydrophobic membranes. The secret ingredients to making it work are two-fold.
First, the design of the rebreather head itself keeps all the sensors very dry, drier than any rebreather I have used before. Secondly, the real magic is not the sensor, it is the computer algorithm that goes with it and matches the unit’s engineering. Moisture looks an awful lot like CO2 to a sensor and the algorithm also has to compensate for density changes of various gas mixtures. The sensor system has been tested and approved by third party CE validation.
Don’t ever assume you can take a single component of any rebreather and put it in another one and expect the same results. - Jill Heinerth